Sulfur is present in the environment in several different forms which may be generally classified as organic and inorganic sulfur. The presence of each form of sulfur has important environmental implications. For example, combustion of both inorganic and organic sulfur produces sulfur dioxide, a greenhouse gas and source of acid rain. Another significant environmental problem is the acid generating potential of a class of inorganic sulfur materials known as reduced inorganic sulfur. These compounds produce acid when materials containing them are mined, excavated or drained, and exposed to atmospheric oxygen. The term “reduced inorganic sulfur” refers to sulfur in a form that can undergo oxidation and includes mineral disulfides (e.g. pyrite and chalcopyrite), monosulfides (e.g. sphalerite, galena and covelline), polysulphides (e.g. pyrrhotite and bornite), non-stoichoimetric metal sulfides (e.g. greigite), other sulphides such as chalcocite, sulfites (salts of sulfurous acids) and elemental sulfur. In the present specification and claims the term “reduced inorganic sulfur” will be understood to include any inorganic sulfur compound that can be oxidised.
Further types of naturally occurring inorganic sulfur compounds are the sulfate minerals, such as gypsum, which include sulfur in the oxidized sulfate form. These materials are generally not a source of acid in the environment.
When sulfide bearing material is mined or excavated, oxidation of the reduced inorganic sulfur may occur. The result is acid sulfate soil and acid mine drainage. Acid mine drainage refers to acid water produced by the oxidation of minerals such as pyrite in the presence of water and is one of the major environmental problems facing the mining industry. The oxidation of reduced inorganic sulfur in acid sulfate soils is a global environmental problem affecting more than 12 million hectares of agricultural land worldwide, degrading aquatic habitats and corroding concrete and steel infrastructure. It is therefore important for environmental management to be able to measure the amount of reduced inorganic sulfur in a wide range of materials including sediments, soils, mine spoil, sludge, petroleum, mineral ores, fossil fuels and water.
There are several known methods for measuring the sulfur content of a material. Combustion followed by measurement of evolved sulfur oxides is used to measure the total sulfur content (i.e. the combined inorganic and organic sulfur). Such methods are typically used to measure the sulfur content of coal. Combustion of coal is a major source of sulfur dioxide pollution. However, the combustion method does not distinguish between organic sulfur, reduced inorganic sulfur or mineral sulfates and accordingly cannot be used to accurately quantify the reduced inorganic sulfur in a sample.
The most widely used method for assaying reduced inorganic sulfur is by peroxide oxidation in which the sulfur is oxidised to sulfate. The amount of sulfate liberated by peroxide oxidation is then analysed by conventional wet chemical quantitative analysis. Although this method is the accepted procedure, the present inventors have surprisingly and unexpectedly observed that the method is subject to serious interferences from organic sulfur and sulfate minerals such as gypsum. This interference is particularly important when sediments having low reduced inorganic sulfur are measured. An erroneous estimate of the reduced inorganic sulfur content may lead to the recommendation of costly and/or inappropriate and environmentally damaging management practices.
Other methods for analysing reduced inorganic sulfur, but which are considered to be less accurate than the peroxide oxidation method, include:    (a) measuring the total sulfur and soluble sulfur content and estimating the reduced inorganic sulfur content from the difference between the two values. A disadvantage of such differential measurement is that the errors are cumulative.    (b) microscopic examination of a sample, and    (c) indirect measurement by estimating the amount of pyritic iron (FeS2) in a sample. However non-pyritic forms of reduced inorganic sulfur are not measured.
It is therefore an object of the present invention to provide a method and apparatus for measuring the reduced inorganic sulfur content of a sample selectively and accurately.